Microporous membranes can be used as battery separator film (“BSF”) in, e.g., lithium ion batteries. Large-capacity batteries such as those that can be used to power electric vehicles and hybrid electric vehicles could be improved by increasing the BSFs meltdown temperature, puncture strength, and electrolyte affinity without significantly decreasing other important membrane properties such as porosity, permeability, and thermal stability (heat shrinkage). Increasing strength is important because it reduces the risk of internal short circuits resulting from BSF failure. Improved electrolyte affinity leads to improved battery manufacturing yield, resulting from the decreased time needed to equilibrate the electrode-electrolyte-BSF assembly.
Microporous membranes comprising polymer have been used as BSFs in lithium ion batteries to provide a fail-safe feature at elevated battery temperature. Such membranes have increased polymer mobility at elevated battery temperature, which leads to a significant permeability decrease. This effect (called “shutdown”) is beneficial because the BSF's diminished permeability above the shutdown temperature results in a decrease in battery electrochemical activity, thereby lessening the risk of battery failure under overcharge, rapid-discharge, or other high-temperature battery conditions. Since battery internal temperature can continue to increase even at reduced electrochemical activity, it is desirable to increase the BSF's thermal stability at elevated temperature to further lessen the risk of battery failure. One measure of thermal stability, the BSF's meltdown temperature, is related to the maximum temperature at which the BSF is able to electrically separate the battery's anode and cathode. A BSF's meltdown temperature can be increased by including a high melting-point species (e.g., polypropylene) in the BSF's polymer to increase the BSF's meltdown temperature.
Monolayer BSFs comprising (i) polymethylpentene and polyethylene and (ii) polymethylpentene and polypropylene have meltdown temperatures ≧200° C., but these films have a lower pin puncture strength and less electrolyte affinity than BSFs comprising polyethylene and/or polypropylene.